In seismic exploration, geophysical data are obtained by applying acoustic energy to the earth from an acoustic source and detecting seismic energy reflected from interfaces between different layers in subsurface formations. The seismic wavefield is reflected when there is a difference in acoustic impedance between the layers on either side of the interface.
Marine seismic prospection is generally made with seismic streamers which are towed through water behind a recorder vessel at a water depth normally between about six to about nine meters, but can be towed shallower or deeper. The streamers support sensors such as hydrophones to detect seismic signals corresponding to pressure waves. Seismic sources may be also towed behind the recorder vessel. Seismic sources may be for example air gun arrays or water gun arrays or other sources known to those skilled in the seismic art.
Alternatively the seismic streamers are maintained at a substantially stationary position in a body of water, either floating at a selected depth or lying on the bottom of the body of water, in which case the source may be towed behind a vessel to generate acoustic energy at varying locations or the source may be maintained in a stationary position.
Multi-component streamers usually use at least two nearly co-located sensors (or group of sensors), one pressure sensor (hydrophone), or a group of pressure sensors and at least one particle motion sensor (geophone or accelerometer) or a group of particle motion sensors.
The at least one particle motion sensor (or the particle motion sensors group) is nearly collocated to the pressure sensor (or pressure sensor group).
While the hydrophone is an omnidirectional sensor and so, does not need to be oriented, the particle motion sensors measure the amplitude of the wave (speed or acceleration of the particle) on a given direction. To do so, the sensors orientation must be known.
Knowing that it is nearly impossible to predict the rotation of the streamer in water, there are usually two possible solutions to know said given direction.
A first solution consists in mechanically insuring that the particle motion sensor(s) is in a known orientation using for example gravity. One way to perform this is to ballast the sensor and gimbal mount the sensor in a housing filled with lubricant damping fluid.
A second solution is to create a 2 or 3-dimension particle motion sensor base and to use a co-located tilt sensor, with a known orientation compared to this base. The tilt measurement is then used to recover the vertical, crossline, or the inline component of the particle motion wave. This can for example be implemented through a MEMs device, that can measure at the same time the tilt and the acceleration.
The first solution has the main disadvantage of affecting the particle motion sensor response, as the motion of the sensor induced by cable rotation is biased by the gimbal arrangement (inertia, friction, etc.). Moreover, such gimbal mounting is usually complex by involving additional mechanical parts and take too much space in the cable.
The second solution solves the issues described above, but it has the drawback of requiring an additional sensor at the sensor location and its associated power. This means more wires in the cable and so, some impact on the overall weight and size of the cable. Furthermore, when this second solution is implemented with a MEMS accelerometer, this solution does not allow to design an analog sensors group, that is necessary to achieve good noise performance without impacted the necessary data rate to get the data back to the boat.
Non limitative examples of known sensors for seismic streamers may be found in prior art documents US 2011/0310698, WO 2011/162799, US 2007/0036033, U.S. Pat. No. 5,675,556 and U.S. Pat. No. 5,541,894.